The breakable-stem blind rivets presently on the market in general are consisting of a shell having a closing head which is inserted on the side of the workpieces facing the setting tool, wherein from the side of the future blind side a mandrel is inserted into the shell, the head of which generally is corresponding to the exterior diameter of the shell. During the setting operation, the rivet is inserted into an opening through both workpieces to be connected until the closing head is contacting the front workpiece. Thereafter, the mandrel is pulled, whereby behind the workpieces to be connected a so-called blind head or closing head is formed.
Such breakable-stem blind rivets according to the prior art unfortunately only have a rather limited grip range. This means, with such a breakable-stem blind rivet of a given size only workpieces with a relatively small difference in their entire thickness can be connected. Further, the shells for the breakable-stem blind rivets according to the prior art are rather complicated to produce because each the closing head has to be formed. In addition with the usual breakable-stem blind rivets, the workpieces to be connected before the beginning of the setting operation have to be compressed.
It is therefore the task to be solved by the present invention to provide for a breakable-stem blind rivet having a larger grip range, wherein preferably in addition it should be achieved that the shell of the rivet is more simple to produce and that workpieces can be connected which not directly contacting each other before the setting operation and which are pressed together by the setting operation.
According to the invention, this task is solved by the feature that the shell before the beginning of the setting operation has an external diameter which at all points is smaller than the internal diameter of the hole in which the breakable-stem blind rivet is set. By this according to the invention a blind rivet is proposed having no closing head. Only when setting the blind rivet according to the invention on both ends of the shell of the blind rivet closing heads are formed. This has the advantage that the production of the corresponding shells is far more simple (these are consisting basically only of a continous small tube), and when using a suitable setting apparatus, the workpieces can be riveted together even if they still are having a certain distance to each other before the setting operation and are only pulled together by the setting operation.
To achieve an extremely good formation of the closing heads, an especially preferred embodiment according to the invention it is provided that the shell on both sides of a central section is having areas, in which the shell is more easily deformable than in the central section.
The axial length of the central section herein is corresponding the minimal grip range of the breakable-stem blind rivet.
Such areas, which are more easily deformable, can be achieved in that the shell on both ends has a conically tapering shape.
In this connection it is further preferred to limit the central section by two circumferential grooves, the distance of which to each other is corresponding to the minimal grip range. By this feature, too, the formation of the closing heads is improved.
In an alternative embodiment according to the invention, in an especially preferred mode, the sections being more easily deformable could be formed by a microstructural change of the material of the shell. This has the special advantage, that merely a microstructural change in limited areas on both sides of the central section has to be performed such, that any additional working of the shell can be dispensed with which means that the shell can be produced with parallel exterior walls in a smoothed shape which simplyfies additionally the method of production substantially.
For performing the microstructural change, in an advantageous way, the more easily deformable areas can be produced by induction annealing of corresponding sections of the shell or alternatively by laser annealing.
In an advantageous improvement according to the invention, it is provided that in the central section there is provided a section having a smaller interior diameter and therefore increasing the wall thickness of the shell, the diameter corresponding to the diameter of the mandrel and, that the mandrel in a section adjacent to his head is having a larger diameter, wherein at the transition between the section of the mandrel having the larger diameter and the remainder of the mandrel, there is provided a groove. By this, the advantage is achieved that the central section, which is more difficult to deform, on the one hand, is defined even clearer and, that, on the other hand, after setting the part of the mandrel remaining in the breakable-stem blind rivet securely is fastened by the fact that by means of the section of the mandrel having the larger diameter during the setting of the rivet material from the section with the increased wall thickness is deformed into the groove.
According to the invention, there further is proposed an apparatus for setting breakable-stem blind rivet having the above features, the apparatus having a mouth piece receiving one end of the mandrel in a gripping apparatus by the means of which the mandrel can be pulled with respect to anvil for the shell contained in the mouth piece in the direction of the front side of the workpieces. According to the invention, here it is proposed that the mouth piece is surrounded by a workpiece pressing means, that the workpiece pressing means is relatively movable with respect to the mouth piece and, that by the means of the workpiece pressing means a force acting on the front side of the workpiece can be created. By this apparatus, it is achieved that with the corresponding size of the force, which can act on the near side of the workpiece, given, firstly, the closing head on the blind side is formed while simultaneously the workpiece on the blind side is pulled onto the workpiece on the near side in case the two workpieces before the setting operation were not directly contacting each other. After finishing the formation of the closing head on the blind side and the possibly occurring movement of the workpiece on the blind side, the force created by the gripping means becomes larger than the force acting on the front side of the workpiece by the workpiece pressing means such, that thereafter the closing head on the front side is formed.
With regard to the construction, it is especially preferred in this connection that the mouth piece on the exterior is having a cylindrical shape and is coaxially surrounded by the workpiece pressing means.
In one embodiment of the apparatus according to the invention, the workpiece pressing means is in a controlled manner advancable against the front side of the workpiece and is retractable up the height of the mouth piece. To set a breakable-stem blind rivet with an apparatus according to this embodiment, the workpiece pressing means firstly is advanced such, that the workpiece is contacting the same and thereafter the mandrel is pulled by the gripping means. During this action, the pressing means is relieving the front end of the shell such, that firstly the closing head on the blind side is formed and thereafter the workpiece on the blind side is pulled onto the workpiece on the front side. Thereafter the workpiece pressing means slowly is retracted in the direction of the mouth piece such, that the anvil of the mouth piece is exerting an increasing pressure on the shell of the breakable-stem blind rivet, whereby thereafter the closing head on the front side is formed.
In alternative embodiments of the apparatus according to the invention, the workpiece pressing means is consisting of an elasticly resilient member pressing against the front side of the workpiece, the pressing force of which is chosen such, that corresponding to the resistance of deformation of the ends of the shell, firstly, the closing head on the blind side and only thereafter the closing head on the front side is formed.
In these embodiments, the workpiece pressing means can be consisting of an elastic body of rubber, hard rubber or the like, or can be prestressed in the direction of the front side of the workpiece by a saucer spring or coil spring.